This invention relates to an image forming apparatus such as a digital copying machine for creating an image signal by reading an image of a document or the like by use of a scanner, for example, and then forming an image on paper according to the image signal by use of a laser printer.
Generally, in a digital copying machine or the like dealing with image information, an image signal read by use of reading means such as a scanner is converted into a proper form, for example, multi-density form depending on the service objective. At this time, if the entire portion or part of the read image is too light or too dark, an input image is subjected to a process for adjusting the density characteristic and then output by use of a laser printer. Conventionally, the adjustment of the density characteristic is generally made by use of a conversion table in which the relation between the density of the input image signal and the corresponding density of the output image signal is set.
The adjustment method using the conversion table necessitates a memory of a large capacity as a memory for storing conversion tables as the number of types of the conversion tables increases and the number of multi-density levels of the image increases. If the multi-density data is expressed by eight bits, it is necessary to store 8-bit values obtained after conversion and used for the adjustment for each of 256 densities which can be represented by eight bits in order to adjust the density characteristic of the image signal.
Therefore, one conversion table necessitates a capacity of 8 bits.times.256, and if a plurality of conversion tables are used, a memory of extremely large storage capacity is required. For example, when the density adjustment is made by use of a manual density setting button for each mode such as a photographic mode or character mode, a memory of extremely large storage capacity is required.
Further, a method using a circuit for deriving values to be stored into the conversion table on the real time basis instead of previously storing the conversion tables in the memory is provided. When the linear conversion is made, a table value deriving circuit used for this purpose can be made relatively small in scale and simple in construction, but if the conversion is made according to a complicated curved line or a curved line which cannot be expressed by an equation, a circuit complicated in construction and large in scale is necessary or the conversion cannot be attained by use of a circuit.
Further, generally, values actually converted by use of the conversion table are not values which are linearly changed before and after the conversion, and they vary along a curved line expressed by a complicated equation or cannot be expressed by an equation in most cases.
In addition, when the density adjustment is made by use of values stored in the conversion table, the content of the adjustment is limited to a range defined by the values in the conversion table and the degree of freedom of the density adjustment becomes low.
An object of this invention is to provide an image forming apparatus in which the storage capacity of a memory for storing density adjustment tables can be reduced and the density adjustment can be performed with a relatively high degree of freedom.